SPARK-1119 and other build improvements

1. Makes assembly and examples jar naming consistent in maven/sbt.
2. Updates make-distribution.sh to use Maven and fixes some bugs.
3. Updates the create-release script to call make-distribution script.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #502 from pwendell/make-distribution and squashes the following commits:

1a97f0d [Patrick Wendell] SPARK-1119 and other build improvements
Merge branch 'master' into pyspark-inputformats
Recover earlier changes lost in previous merge for context.py
Recover earlier changes lost in previous merge for serializers.py
Add back context.py changes